I remember trying this ioquake3 mod on N900 over a year ago. It's apparently a different version and it was slow. However this may work, what I did with the ioquake3 version 1558-maemo5 was:

add the folder containing mod files into the hidden .q3a folder and the mod appears in the Quake 3 "Mods" list, with the name of the mod folder.

Start it with X-terminal because ioquake3 crashes: